142 
SILUEIA. 
[Chap. VII. 
its organic remains that the lowest of the bone- beds is really part and 
parcel of the Ludlow rock *. 
In the next chapter it will, indeed, be seen that large and peculiar 
species of Crustaceans (Pterygoti) also specially characterize the very up- 
permost beds of the Silurian rocks of Scotland, or the black slaty shale of 
Lanarkshire, which clearly underlies every stratum to which the term Old 
Bed Sandstone has been applied. Again, as will afterwards be shown, the 
same term in the series of the Palaeozoic rocks of Eussia, viz. grey and 
calcareous strata ud deriving all the Devonian rocks, is characterized by 
Crustaceans of the family of Eurypteridaa ; whilst both in Scotland and 
Eussia these Crustaceans are associated with Lingula cornea, an unmis- 
takeable shell of the Upper Ludlow rock. 
The formations and subdivisions of the Silurian rocks of the region 
illustrated in the Map having now been described in the greatest detail 
compatible with the limits of this work, the reader is referred to the 
accompanying valuable Table of comparison, prepared by Mr. Aveline of 
the Geological Survey, that he may recognize at one view the varied de- 
velopments of the strata in different tracts of Wales and the adjacent 
English counties. 
The first of the vertical columns in this Table represents a more complete 
series than can be seen in any one consecutive natural section. It is a 
union of all the known strata, as derived from various localities. A near 
approach, however, to this complete sequence of beds is observable in 
South Wales, as expressed in the second column, which country, including 
the neighbourhood of Llandovery (No. 4), together with Shropshire, Here- 
fordshire, and the border Welsh tracts of Montgomery and Eadnor (Nos. 5, 
6, 7, 8, & 9), constituted the original Silurian region. By observing this 
comparative view, we mark the attenuation, in some districts, of deposits 
which are expanded and interpolated in others, also that certain bands 
well exposed in limited areas are wholly omitted in other tracts. The 
inquirer has thus before him the condensed results of the long and labo- 
rious researches of the Geological Survey in unravelling the highly diver- 
sified conditions of the substrata in Wales and Siluria. 
In viewing this Table, it is to be recollected that in column No. 8 the 
ascending order might have been continued through the Ludlow rocks up 
to the Old Bed Sandstone, inasmuch as that succession is everywhere 
visible to the S.E. of the Wenlock Edge. By combining such an ascending 
* Mr. D. Page, in his able Manual, has re- Old Eed Fishes) in the lowest bone-bed affords 
garded the original Ludlow bone-bed as part of no reason for separating it from the Ludlow rocks, 
the Old Eed Sandstone. His ground for this seeing that this very band is filled with marine 
classification is, that fossil Fishes here first appear. Shells unequivocally of Silurian age, many of 
But I contend that the presence of Plectrodus which are found low down in the system, 
mirabilis and Onchus Murchisoui (neither of them 
